When the Jobs ages out or the pages exceeds beyond the applied parameters the jobs turns out blank
By default Netvault limits the logs truncation as a part of archival and housekeeping and controlled through the logging daemon parameters.
1. Increase the Auto Purge Method to convenient dates where you can review them and increase the page limit
Manage setting | Logging Daemon
Auto-purge method - Exceeds log age
Auto-purge when the number of log pages exceeds - default 10
2. Alternately you can Run reports to fetch information